Nearly 500 isotopes are produced in nuclear reactors by fission and irradiation processes. Current options for dealing with high-level wastes resulting from operation of fission reactors (e.g., nuclear power generation) include separation of long-lived uranium and plutonium from other high-level wastes using some form of the PUREX (PlutoniumURaniumEXtraction) Process. The volume of the aqueous effluent from the PUREX process containing the remainder of the high-level wastes, usually referred to as fission products, must then be reduced, and the residue solidified and eventually buried in a suitable repository. The high-level fission products that remain in the aqueous phase subsequent to the PUREX process include .sup.137 Cs.sup.+ and .sup.90 Sr.sup.+2. These radionuclides have potential commercial value and/or are problematic in subsequent disposal schemes. It has been estimated that 30 years after reactor discharge, .sup.137 Cs.sup.+ and .sup.90 Sr.sup.+2 account for 98% of the thermal energy and 97% of the penetrating radiation in high-level waste, and dominate the design restrictions of waste repositories. Removal of those two nuclides would have the same effect as aging the waste hundreds of years. Fission-product cesium in freshly discharged spent fuel consists of the stable isotope .sup.133 Cs and the radioactive isotopes .sup.134 Cs (t.sub.1/2 =2.1 years), .sup.135 Cs (t.sub.1/2 =2.3.times.10.sup.6 years), .sup.136 Cs (t.sub.1/2 =13 days) and .sup.137 Cs (t.sub.1/2 =30.0 years), with .sup.137 Cs accounting for about 43% of the total cesium. Fission-product strontium in freshly discharged spent fuel consists of the stable isotope .sup.88 Sr and the radioactive isotopes .sup.89 Sr (t.sub.1/2 =50.5 days), .sup.90 Sr (t.sub.1/2 =29.1 years) and .sup.91 Sr (t.sub.1/2 =9.5 hours), with .sup.90 Sr accounting for about 60% of the total strontium.
Various systems have been explored for the large-scale separation of .sup.137 Cs.sup.+ and .sup.90 Sr.sup.+2 from aqueous wastes including: (1) adsorption followed by precipitation, and (2) extraction. The present invention is directed to extraction agents and processes.
The extraction of rubidium and cesium ions into nitrobenzene using aqueous solutions of tetraphenylborate was reported in 1956, in a thesis by a student at MIT. A few years later, government reports suggested extraction as a viable technique for the removal of cesium ion from nuclear fission waste. See, Smith et at., USAEC HW-76222 (1963) (cited in J. Inorg. Nucl. Chem., 30:611 (1968); Bray et at., USAEC HW-76222 (1963), and Richardson, USAEC HW-75447 (1963).
In the late 1960's and early 1970's, several researchers studied the extraction of alkali metal ions and alkaline earth metal ions from radioactive waste using a variety of agents, including dipicrylamine, pierate, tetraphenylborate, catechol, and catechol derivatives. The extraction of cesium and other ions into various polar organic solvents, such as isoamyl alcohol, nitrobenzene, nitromethane, nitroethane, methylisobutyl ketone, and tdbutylphosphate was also reported.
The driving force for the extraction of cations from the aqueous phase into the organic phase is thought to be relief of the disruption of hydrogen bonding in the water "structure" caused by the presence of large, non-hydrated or partially hydrated, cations of low charge (e.g., Cs.sup.+). The selectivity and efficiency of extraction from the aqueous phase to the nitrobenzene phase is lower for alkaline earth ions (e.g., Sr.sup.+2), than for alkali metal ions. Thus, the proposed order of extractability is Ca.sup.+2 &lt;Sr.sup.+2 &lt;Li.sup.+ &lt;Na.sup.+ &lt;NH.sub.4.sup.+ &lt;K.sup.+ &lt;Rb.sup.+ &lt;Cs.sup.+.
All of the anionic phase transfer agents discussed above decomposed, existed in more than one form in relatively strong nitric acid media, or in some other way were less than satisfactory. For example, in strongly acidic and radioactive extraction environments, tetraphenylborate ion decomposes to carcinogenic benzene, which can complicate subsequent processing.
During the early 1970's, it was reported that some polyhedral boranes could be separated from aqueous systems containing other ions by ether extraction. Extractability was shown to increase with the size of the polyhedron, e.g., B.sub.3 H.sub.8.sup.- &lt;B.sub.9 H.sub.14.sup.- &lt;B.sub.11 CH.sub.12.sup.- &lt;Co(B.sub.9 C.sub.2 H.sub.11).sup.-2 (cobalt dicarbollide ion).
The first transition metal dicarbollide ion complex (e.g., iron dicarbollide, Fe(B.sub.9 C.sub.2 H.sub.11).sub.2.sup.-) was reported by Hawthorne, et al. in 1965. Previously, Hawthorne et al. had reported the degraffation of carborane (B.sub.10 C.sub.2 H.sub.12) to dicarbadodecahydroundecaborate (-1) ion (B.sub.9 C.sub.2 H.sub.12.sup.-), the precursor of dicarbollide ion (B.sub.9 C.sub.2 H.sub.11.sup.-2).
The 1,2-dicarbollide ion (1,2-B.sub.9 C.sub.2 H.sub.11).sup.-2 and a transition metal bis-dicarbollide ion complex M(B.sub.9 C.sub.2 H.sub.11).sub.2.sup.-, is sometimes referred to as a metal dicarbollide ion (or metal dicarbollide complex).
The numbering system for the atoms in the cage structures of isomeric dicarbollide ions and bis-dicarbollide transition metal complexes in most of the references cited is the "old system" in which the carbon atoms occupy positions 1 and 2 or positions 1 and 7, and the transition metal atom occupies position 3. To avoid confusion with the bulk of the pertinent references, the old numbering system is used herein.
To date, several metal dicarbollide ions have been made, including iron. dicarbollide ion, chromium dicarbollide ion, cobalt dicarbollide ion, and nickel dicarbollide ion. The chemistry of these complexes and some of their derivatives has been reviewed several times. See generally, Callahan and Hawthorne, "Ten years of metallocarboranes," Adv. Organomet. Chem. 14:145-86 (1976).
Although the sodium salt of cobalt dicarbollide is a strong electrolyte, the fact that it can be extracted into ether from aqueous solution led to extensive studies of cobalt dicarbollide ion as a phase-transfer agent, including examination of its use as an extraction agent for radionuclides. The following is a representative list of some of the patents and other references that have been published about this topic:
Czech Patent No. 153933 (Ralsetal.) and J. Inorg. Nucl. Chem. 38(7):1376-8 (1976) describe the extraction of radionuclides from aqueous solutions using cobalt dicarbollide ion, and reported that, because of the solubility of cesium cobalt dicarbollide in various water-immiscible solvents and the distribution ratios of the cobalt dicarbollide ion in the organic phase relative to the aqueous phase, only polar solvents like nitrobenzene and some chlorinated solvents were viable extraction media.
Scasnar and Koprda, in Radiochem. Radioanal. Lett. 34(1):23-8 (1978), reported the extraction of 137Cs.sup.+ into nitrobenzene in approximately 99% yield using the protonated form (conjugate acid) of unsubstituted cobalt dicarbollide ion. Subsequent to extraction, aqueous acids (&gt;2M) effectively strip the Cs.sup.+ ion from the nitrobenzene phase.
Sr.sup.+2 ion can be similarly extracted, but requires the addition of a polyethylene glycol (e.g. PEG-400) or p-nonylphenyl-nonaethylene glycol (e.g., Slovafol-909). Czech Patent No. 153933 (Raiset al.) and Collect. Czech. Chem. Commun. (Rais et al.) 44(1):157-66 (1979) describe the extraction of Sr.sup.+2 from aqueous solvents using metal dicarbollide ion and polyethylene glycol. It is believed that the polyethylene glycol forms a complex with Sr.sup.+2 through the oxygen atoms of PEG. The hydrocarbon part of PEG is directed outward, making the complex more hydrophobic than the bare ion and thus more easily extracted into an organic solvent.
In general, for the two-phase water-nitrobenzene extraction system, radionuclide extraction selectivity and efficiency does not depend on the cobalt dicarbollide derivative employed. However, although the stability of unsubstituted cobalt dicarbollide ion in nitrobenzene toward HNO.sub.3 decreases markedly above 2M HNO.sub.3, the dibromo- derivative is quite stable to 5M HNO.sub.3. Selucky et al., in Ustav Jad. Vyzk. [Rep.], UJV 5069 (1979), reported that both chlorination and bromination of boron atoms in the dicarbollide ion cage increases the stability of cobalt dicarbollide ion. Electrophilic substitution on boron atoms of the dicarbollide ion cage was reported as early as 1968 by Hawthorne and co-workers. In the mid-1970's, researchers in Czechoslovakia described the dibromination of cobalt dicarbollide complexes in 20% mixtures of bromoform in nitrobenzene, and hydroxylation in chloroform or carbon tetrachloride-saturated aqueous solution under radiation conditions. Direct bromination with bromine in methanol produced the 8,8'-dibromo derivative.
Matel et al. (Radiochem. Radioanal. Lett. 46(1-2):(1-6) (1981) attributed the improved chemical and radiation stability of the brominated products to blockage of the reaction centers, and Coursel et al. (Can. J. Chem. 64(9):1752-7 (1986)) determined the regions of greatest electron density (boron atoms in the 8 position).
Czech Patent No. 242501 describes the 8,8'-dichlorination of transition-metal dicarbollide complexes, including Co(III), Fe(III) and Ni(III). Subsequently, Matel et al., reported in Polyhedron 1(6):511-19 (1982), the chlorination and bromination of cobalt dicarbollide by elemental halogen in alcohols, and .gamma.-induced halogenation by bromoform, chloroform or carbon tetrachloride in polar solvents. Halogenation was reported to proceed alternatively in both cages yielding, successively, 8-; 8,8'-; 8,9,8'-, 8,9,8',9'-, 8,9,12,8',9'- and 8,9,12,8',9',12'-halogen derivatives.
Czech Patent No. 224890 (Rais et al.) reports the preparation of the hexachloro- derivative of cobalt dicarbollide ion, using elemental chlorine in acetic acid, on the 14 kg scale.
Extraction studies similar to those previously done using unsubstituted cobalt dicarbollide ion were repeated using various halogenated derivatives. For example, Czech Patent No. 224890 reports the extraction of Sr.sup.+2 using the hexachloro-derivative, and Czech Patent Nos. 219525 (Kyrs et at.) and 224993 (Macasek et al.) report the separation of Cs.sup.+ and Rb.sup.+ ion using the dibromo- derivative.
Scasnar et al., in Radiochem. Radioanal. Lett. 50(6):333-43 (1982) reported a chromatographlc procedure for separating Cs.sup.+ ions from a fission product mixture, using KEL-F (poly-trifluorochloroethylene) as solid support for cobalt dicarbollide ion or a chlorinated derivative in nitrebenzene.
Selucky et al., in Radiochem. Radioanal. Lett. 38(4):297-302 (1979), described the development of procedures for the initial separation of Cs.sup.+ ion with 0.01M cobalt dicarbollide ion followed by extraction of Sr.sup.+2 ion with a "hydrophobizing" agent (PEG). The most recent Eastern European work centers around fine tuning the extractions by solvent variations and/or by the addition of "synergetic" phase transfer agents. Thus, Selecky et al., in J. Radioanal. Nucl. Chem., 148(2):227-33 (1991) reported the extraction of fission products from nitric acid media using 1,2-dichloroethane solutions of the hexabromo- derivative of cobalt dicarbollide ion. Babain et al., Radiokhimiya, 35(2):81-5 (1993), proposed diethyleneglycol-bis-(tetrafluoropropyl) ether as an alternative solvent to nitrobenzene for extraction of Cs.sup.+ ion and Sr.sup.+2 ion with chlorinated cobalt dicarbollide ion and Slovafol-909. Ivanovetal., Sib. Khim. Zh., (4):78-81 (1991), reported that toluene/1-octanol and toluene/butyl acetate mixtures may be capable of extracting Cs.sup.+ with very little dependence of the extraction efficiency on solvent composition.
Tran et at., in Nucleon (2):6-9 (1993) reported the separation of Sr.sup.+2 from Ca.sup.+2 using an organic phase that contained 0.11M lithium cobalt dicarbollide and 1.7% Slovafol-909 in a solution of 40% carbon tetrachloride/60% nitrobenzene. Their studies suggested that the efficiency of alkaline earth extraction increased with the number of ethylene oxide units in the range 2 through 8.
Zakharldn et al., Plast. Massy (11):61-3 (1990) reported the crosslinking of polystyrene using cobalt dicarbollide ion, and proposed it as a sorbant for separation of fission products.
El Said et at., J. Radionanal. Nucl. Chem. 163(1):113-21 (1992) described an "Emulsion Liquid Membrane" system of di-2-ethylhexylphosphoric acid, n-alkane, dipicrylamine and cobalt dicarbollide ion in nitrobenzene stabilized with surfactant (SPAN 80/85), and its use to separate radioactive Cs.sup.+ ion, Sr.sup.+2 ion, Ce.sup.+3 ion and Eu.sup.+3 ion from acidic nitrate solution.
The field of fission products extraction was reviewed by Schulz et at., in Sep. Sci. Technol. 22(2-3):191-214 (1987). See, also, Rais et al., Nucleon 1:17 (1992), and Nukleon (4): 13-16 (1987).
Other agents and methods for extracting cesium and strontium from aqueous nuclear waste are known and/or are being studied. For example, calixarenes have been studied as alkali metal ion complexing agents, and in the extraction of cesium ions from aqueous waste. Their subsequent decomposition has also been studied.
Similarly, crown ethers are well known complexing agents for metal ions and can be tailored to "fit" ions of different sizes. Both Cs.sup.+ and Sr.sup.+2 have been extracted from aqueous systems using crown ethers. Crown ether terminated polysiloxanes in supported liquid membranes have been studied with regard to potassium ion transport. Alkali metal ion transport across polymer membranes has also been studied.
To date, the known technologies for removing high-level fission products (specifically, .sup.137 Cs.sup.+ and .sup.90 Sr.sup.+2 ions) from the strongly acidic aqueous effluent generated in most reprocessing procedures have used (1) solvent and complexing agent-intensive zeolite adsorption; (2) environmentally unfriendly solvents such as nitrebenzene and chlorinated solvents as the immiscible organic phase; or (3) agents, such as calixarenes and crown ethers, that tend to be capable of efficiently separating only one of the two fission products of interest. Calixarenes and crown ethers both suffer from the drawback that, because of the strong bonding interaction between metal ions and the oxygen atoms of the complexes, stripping the metal ions from the complexes is often difficult or impossible without destroying the complexes.
What is needed is a system capable of selectively and efficiently extracting both .sup.137 Cs.sup.+ and .sup.90 Sr.sup.+2 from high-level fission product waste in a single, integrated process that avoids nitrebenzene, chlorinated solvents, and other drawbacks encountered with known fission product extraction processes.